1. Field of the Invention
The invention relates to a method and a device for manufacturing a metallic hollow body having at least one hollow space, particularly a turbine blade having a cooling air channel and multiple cooling air openings.
2. Related Art
Various methods are known in the art for manufacturing metallic hollow bodies comprising a hollow space where the casting methods play a special role. Casting methods permit the production of precise, fully dimensioned components where the component is substantially shaped in one step, during the casting process, and merely some processing steps for fine machining may possibly be required. Consequently, such casting methods are particularly suitable for manufacturing turbine blades, particularly gas turbine blades. In order to be able to consistently withstand the high temperatures in operation turbine blades are metallic hollow bodies, for example, whose hollow space is designed as a cooling air channel which can be acted upon with cooling air. Turbine blades with a so-called film cooling additionally have cooling air openings on their outside surface leading into the cooling air channel and forming a cooling air film on the outside surface of the turbine blade for the purpose of cooling.
DE 38 23 287 C2 specifies a casting method where a core forming the hollow space is encompassed by a wax jacket. The thickness of the wax jacket corresponds to the thickness of the wall of the component to be cast. Pins are inserted into the wax jacket whose inside ends touch the core while the outside ends of the pins project above the wax jacket. The wax jacket with the pins is then dipped into a ceramic paste, encompassed by the latter and subsequently heated so as to allow the ceramic paste to harden and form a ceramic exterior casting mold. During the heating process the wax jacket melts while the core held by the pins remains fixed in its position. The hardened ceramic paste with the core, which is usually also ceramic, forms the casting mold which is subsequently filled with molten metal. The material of the pins, for example platinum, can be melted on by means of the molten metal and diffuse into the metal. The material for the pins is selected such that substantially no localized, harmful alloys will develop. In order to prevent flaws from developing while the metal component solidifies, which may occur as a result of heat loss on the pins projecting into the exterior casting mold, for example, the pins are provided with heat retaining caps that help prevent rapid heat loss on the pins. Cooling air openings leading into the hollow space are subsequently drilled through the exterior wall for producing a film-cooled turbine blade.
One disadvantage of the above method is that the pins extend so far into the exterior casting mold that the ends of the pins will project above the surface of the completed component which requires the component to be reworked. Furthermore, the pins cannot have any desired width for fixing the core in its position because undesirable localized alloys could develop. In addition, the number of pins of platinum for fixating the core has to be limited for cost reasons.
In order to prevent that a finished component has to be reworked, DE 33 12 867 A1 specifies a method where the core forming the hollow space is encompassed by a support whose external shape will not project above the surface of the component to be cast. The core, including the support, is subsequently encompassed by a wax jacket and dipped into a ceramic paste. In this case, the support for the core consists of a material which dissolves in the cast alloy and will not negatively affect the properties of the component.
Again, there is the disadvantage of having to drill cooling air openings into the wall of the turbine blade by means of an additional process step.
Furthermore, the disadvantage of both methods is that already when the wax jacket is removed, the varying thermal expansion behavior of the pins or the support and the core could cause the core to shift relative to the future exterior wall which will result in a fluctuating wall thickness.
It has been found that a casting mold which was produced with the aid of a wax-jacketed core already has deviations in the hollow space with regard to the desired wall thickness of the component to be cast when it is released by the wax. The deviations in the position of the core with regard to its desried position are the result, among other things, of the varying thermal expansion of the ceramic core, the metallic pins or supports and the wax forming the wax jacket.
Further deviations can occur when the hollow space formed by the casting mold is filled with molten metal and during the subsequent solidifying of the metal. The varying thermal effect on the core and the pins or supports of the casting mold can result in a varying thermal expansion, which, under adverse conditions, can cause the core to warp and thus result in a further localized variation in the wall thickness.
It is the object of the invention to provide a method for manufacturing a metallic hollow body It is also the object of the invention to provide a device for manufacturing a metallic hollow body, particularly a turbine blade for a gas turbine.
The problem of finding a device is solved in accordance with the invention by means of a device for manufacturing a metallic hollow body having at least one hollow space and a wall encompassing the hollow space, comprising an exterior casting mold which has at least one inside core serving to form the hollow space, where the exterior casting mold is separable into at least two exterior members and the inside core is connected via at least one connecting element, which serves to form a through-opening in the wall into the hollow space, with an exterior member of the exterior casting mold.